Bo Zhou ¹ Xiu Ying Wei ² Zheng-Yong Wen ^2* Bin Wang ¹ Yu-Ying Zhao ² 万弘 Wanhong ² Yu He ² Panita Prathomya ³ Yunyun Lv ² Yanping Li ² JUN WANG ² Rui Li ² Xu-Guang Li ⁴ Jun Zhou ⁴ Shiyong Zhang ⁴ Jun-De Fan ² Qiong Shi ⁵

• ¹ Sichuan Academy of Agricultural Sciences, Chengdu, Sichuan Province, China
• ² Neijiang Normal University, Neijiang, China
• ³ Chiang Mai University, Chiang Mai, Thailand
• ⁴ Freshwater Fisheries Research Institute of Jiangsu Province, Nanjing, Jiangsu Province, China
• ⁵ Shenzhen University, Shenzhen, Guangdong Province, China

Fatty acid desaturases (FADs) are rate-limiting enzymes for the biosynthesis of highly unsaturated fatty acids (HUFAs). As a new member of the FAD family, Fad6 and its roles remain unclear in various teleost fishes. In this study, we identified a fad6 gene from yellow catfish (ycfad6) and determined its spatiotemporal expression patterns and responses to different initial diets and nutritional status in yellow catfish. Our results showed that the open reading frame (ORF) of ycfad6 was 1,080 bp in length, encoding a protein of 359 amino acids. Multiple protein sequences alignment proved that fad6 is highly conserved among diverse vertebrates. Meanwhile, phylogenetic analysis revealed that Southern catfish and yellow catfish were clustered into one branch, supporting evolutionary consistence between the fad6 gene and fish morphology. Moreover, comparisons of genomic synteny and gene structure revealed functional and evolutionary conservation of the fad gene in various teleost fishes. Tissue distribution analysis by quantitative RT-PCR demonstrated that the ycfad6 gene was extensively expressed in examined tissues, with higher transcription levels in the heart and liver. Meanwhile, ycfad6 gene was widely expressed in various developmental stages, indicating Fad6 may play important roles in HUFA biosynthesis at early developmental stages in yellow catfish. Functional experiments verified that the transcription of ycfad6 decreased significantly with the extension of feeding time (with egg yolk or Artemia nauplii) at the early developmental stages, indicating that a diet rich in HUFA can remarkably inhibit the transcription of ycfad6 in yellow catfish. In addition, ycfad6 transcription was significantly reduced after a short-term (24-h) or long-term food deprivation (1-week) and then continued to decrease during refeeding, suggesting that nutritional states can affect the transcription of ycfad6, which further regulates the metabolism of HUFAs. Anyway, these fundamental findings provide basic references for further investigating evolutionary and physiological functions of the fad6 gene in yellow catfish as well as in other teleost fishes.